I think squadron codes emerged in the RAF only in the late 1970s. Until that time, a tail letter and a squadron badge on the fin (or the cabin door on a helicopter) was deemed sufficient.
I cannot claim to have flown in all the 230 Sqn Whirlwinds in '67-68, but when I flew I logged both serials and tail letters. They were, in no particular order:
XS412(Y), XP396(S), XR453(W), XP299(P), XP357(X), XP395(R), XK986(U), XP329(V), XJ758(O), XP330(Z), XL110(T), and XP302(N).
Hope that helps.